The present invention relates to a liquid crystal electro-optical device using thin-film transistors (hereinafter referred to as TFTs) as drive switching elements in which device the liquid crystal material is a ferroelectric liquid crystal or an antiferroelectric liquid crystal each of which has spontaneous polarization and is superior in high-speed response, or what is called a polymer liquid crystal (also called a dispersion-type liquid crystal) that is a, ferroelectric or antiferroelectric liquid crystal containing a polymer.
In particular, the invention relates to a liquid crystal electro-optical device which provides gradations by controlling a period in which a liquid crystal is optically transparent or non-transparent to express (display) a halftone color or light and shade.
A liquid crystal composition exhibits a different light transmittance and refractive index depending on an external electric field. Utilizing this property, an electrical signal can be converted to an optical signal, which is then used for a display. Among liquid crystal materials are a TN (twisted nematic) liquid crystal, a STN (supertwisted nematic) liquid crystal, a ferroelectric liquid crystal and an antiferroelectric liquid crystal. In recent years, a material called a polymer liquid crystal (also called a dispersion-type liquid crystal) is used, that is a nematic, ferroelectric or antiferroelectric liquid crystal containing a polymer material. It is known that liquid crystals cannot react Co a variation of an external voltage in an infinitely short time, but need a certain time. Each liquid crystal material has its own response time. The response time is several tens of milliseconds in TN liquid crystals, several hundred milliseconds in STN liquid crystals, several tens of microseconds in ferroelectric liquid crystals, and several tens of milliseconds in polymer (dispersion-type) liquid crystals using a nematic liquid crystal.
At the present time, active-matrix type devices can produce the best image quality among display devices using a liquid crystal. Conventional active-matrix type liquid crystal electro-optical devices use thin-film transistors (TFTs) as active elements, and an amorphous or polycrystalline semiconductor is used for the TFTs. Only one of a P-type TFT or an N-type TFT is used in a single pixel. Usually, an N-channel TFT (NTFT) is connected in series to a pixel. Signal voltages are applied to matrix signal lines. When a TFT located at an intersection of signal lines receives both signal voltages, it is turned on. Utilizing this phenomenon, on/off states liquid crystal pixels are controlled individually. By controlling the pixels in this method, a high-contrast liquid crystal electro-optical device can be realized.
However, it is very difficult for the conventional active-matrix type devices to provide a display with gradations that includes light and shade or a halftone color. Conventionally, to enable a display with gradations, there has been investigated a scheme which utilizes the phenomenon that the light transmittance of a liquid crystal varies with the magnitude of a voltage applied thereto.
However, the voltage dependence of the light transmittance of a liquid crystal has very strong nonlinearity. Since the light transmittance abruptly varies at a particular voltage, it may be the case that only a several percent variation of the pixel voltage causes a very large variation in the light transmittance. Therefore, where respective liquid crystal pixels receive different voltages due to nonuniformity in TFTs and a matrix wiring, conventional analog gradation display methods cannot provide gradations more than 16. For example, what is called a transition region (where the light transmittance changes) of TN liquid crystal materials has a range as narrow as 1.2 V. To attain the 16-gradation display, it is necessary to control voltages in a small value of 75 mV. This results in a much lowered production yield. TFTs and a matrix wiring are required to be very uniform.
Because of the difficulty in performing gradation display as described above, liquid crystal display devices have a great disadvantage in competing with CRT (cathode ray tube) displays, which are conventional, generally employed display devices. To solve this problem, the present inventors have found that it is possible to obtain visual gradations by controlling a time during which a voltage is applied to a liquid crystal, as disclosed in detail in Japanese Patent Application No. Hei. 3-169306.
However, in the case of TN liquid crystal displays, the application voltage is required to have accuracy in the same level as in the conventional analog gradation display methods. More specifically, a level "10" as displayed by applying an on-voltage of 5 V to pixels is seen about 2% darker than a level "10", as displayed by applying 5.1 V. Therefore, this type of gradation display method requires TFTs to have as small a variation as the conventional gradation display methods.
As described above, the reason why the pixel voltage needs to be controlled precisely is that a TN liquid crystal changes its light transmittance with the effective value of a voltage applied thereto. The same thing is true also in STN liquid crystals and dispersion-type liquid crystals using a nematic liquid crystal, which is a basic material of the above liquid crystals.
To solve the above problems, there has been proposed, in a liquid crystal electro-optical device that uses thin-film transistors as drive switching elements and performs the above-described gradation display method, a method which uses a ferroelectric or antiferroelectric liquid crystal material, or what is called a polymer liquid crystal (also called a dispersion-type liquid crystal) that is a ferroelectric or antiferroelectric liquid crystal containing a polymer. In this method, gradation display is obtained by controlling a time during which a liquid crystal is in a transparent (light) state or a non-transparent (dark) state. Details are disclosed in Japanese Patent Application No. Hei. 4-333605, for instance.
To obtain transparent and non-transparent states by driving a ferroelectric liquid crystal by an external electric field or the like, it is necessary to reverse spontaneous polarization of the liquid crystal material. Where, as in the case of active driving, the driving method is such that the switching elements are made off during a non-selection period to interrupt external charge supply, it is necessary to consider a relationship between the amount of charge injected into a pixel during a selection period and spontaneous polarization of the liquid crystal material.
In particular, to perform high-resolution gradation display according to the above-described gradation display method, in which case switching between light and dark states is performed in several microseconds (shortest case), the charge supply and the charge consumption due to the reversal of polarization should be balanced very strictly.
In the conventional method, although it is considered to supply sufficient charge for reversing the polarization of the liquid crystal material, no consideration is made of a series resistance between the electrodes, a leak conductance and a pixel charging/discharging time due to a pixel impedance, which factors are important in connection with the charge injection into the electrodes. Therefore, the conventional method has a problem that when it is intended to increase the number of gradations, desired gradation performance is not obtained, for instance, gradation levels become indefinite.